A prior art processing system 100 is shown in FIG. 13 for processing objects in a processing chamber 102 using processing gases, and for drawing effluent waste gases from the processing chamber with a pumping apparatus 104. The process chamber 102 is provided with at least one inlet 106 for receiving one or more process gases from gas sources indicated generally at 108 in FIG. 13. During the processing conducted within the chamber 102, only a portion of the process gases supplied to the chamber will be consumed, and so the waste gas stream exhausted from the outlet 110 of the process chamber 102 will contain a mixture of the unused process gases supplied to the chamber 102, and by-products from the process conducted within the chamber 102.
The pumping system 104 comprises at least one dry pump 112 (a single pump is shown in FIG. 13, although any suitable number may be provided depending on the requirements of the processes carried out in the chamber 100). Each pump (or pumping stage) 112 is a dry pump to ensure that lubricant does not migrate upstream of the pump and contaminate the processing chamber. Also, in general, a dry pump does not provide a substantial source of moisture with which the effluent gases may react and form corrosive products to damage the pump. The dry pump 112 may comprise a multi-stage dry pump, wherein each pumping stage may be provided by a Roots-type or Northey-type pumping mechanism, a turbomolecular pump, and/or a molecular drag mechanism, depending on the pumping requirements of the process chamber 102.
Depending on the deposition step or cleaning step conducted within the process chamber 102, the waste gas stream exhausted from the dry pump may contain one or more silicon-containing or halogen-containing gases which are used as precursors in the manufacture of semiconductor devices. Examples of such gases include hydrogen fluoride, carbon tetrachloride, nitrogen trifluoride, silane, disilane, dichlorosilane, trichlorosilane, tetraethylorthosilicate (TEOS), a siloxane (such as octamethylcyclotetrasiloxane, OMCTS) and the organosilanes. Wane, for example, is typically used as a process gas in the deposition of polysilicon or silicon dioxide layers in a chemical vapour deposition (CVD) process. The gases containing fluorine or chlorine are often used for process chamber cleaning steps.
In a conventional apparatus, effluent gases are conveyed from the dry pump 112 to an abatement arrangement 114 which may contain a combustor and/or a wet scrubber. A combustor may comprise a plasma torch or another flame based device, such as an inwardly fired foraminous burner, which decompose some of the gases in the effluent gas stream. A wet scrubber may comprise a water tower or liquid ring pump through which the effluent gas stream is passed. A component of the effluent gas stream may react with or be soluble in water. Treated gas 116 is exhausted from the abatement apparatus 114.
There are many problems associated with the above described prior art system, some of which are described below.
The reaction kinetics of fluorine with water are dependent upon the temperature, pH, chemical composition of the scrubbing liquid. The reaction products are also affected by these factors. At low temperatures and high pH values significant quantities of OF2 can be generated. This is highly undesirable as OF2 is more toxic than fluorine. To achieve high efficiency of Fluorine scrubbing at lower temperatures without significant OF2 formation the scrubbing liquid must be dosed with a chemical, such as a thiosulphate, and this adds to the cost and complexity of the scrubbing system.
In some processes used in the manufacture of semi-conductors, solar panels and flat-panel displays the exhaust gases contain entrained powder and highly reactive gases which may cause blockages in abatement arrangements.
A semiconductor process dry pump must also be designed to cope with and/or eliminate the accumulation of particulate within the pump. The area most prone to this problem is the exhaust or LV end of the pump, where the pressure is highest and clearances are tightest. Failure to deal with these issues can result in seizure, for example: entrapment of condensable materials in tight axial clearances, entrapment of process particulate in tight axial clearances following dust ingestion, particulate accumulation between close running clearances, and inability to restart because of thermal contraction of the pump stator onto the rotors as the pump cools down.
Current methods used to overcome these issues, include maximising motor starting torque, introducing dust handling features and running at elevated or optimised pump exhaust temperatures to suppress process gas condensation. However, as the new generation of inverter driven pumps run faster, with lower starting torques and with tighter clearances, and as processes steps use more precursor gases, these measures are proving to be less effective.
All dry vacuum pumps are potential ignition sources. Whilst there is no deliberately intended metal-metal contact in the pumping chambers it is possible for the rotor tinting to slip and allow contact. Furthermore, the process by-products which do not pass through the pump can collect or condense, causing contact and hence hot-spots within the pump. An extreme case is when the pump is caused to seize which is a common but random occurrence. There is an increasing requirement within the semiconductor sector for a pump that can pump flammable mixtures.
Typically in vacuum system the upstream foreline connecting the dry pump to the process tool is protected against flame transmission back towards the chamber by maintaining a low pressure (normally less than 60 mbar—but it will depend on the specific process gases). However, there remain concerns around flame transmission into and along the pump exhaust line.
Although the additional of purge gas to dilute any flammable fluids can be effective in reducing or eliminating combustion, high purge gas flows are not popular in the semiconductor industry because of the cost of the gas and the impact on downstream treatment. Exhaust gas abatement is commonly achieved by combustion of the effluent. However, combustion is more difficult to achieve if the gas stream has been heavily diluted to make it non-flammable.
Combustion type abatement systems are also a potential source of ignition of the upstream flammable mixture—the ignition source is constantly present but a flame would have to travel back against the flow.
When stopping a liquid ring pump the vacuum generated in the foreline upstream of the pump must be decreased to prevent service liquid (generally water) from being drawn into the foreline. For many pump applications in the semiconductor and related industries such as flat panel display and solar panel manufacture pump forelines are kept clean and dry due to the reactive nature of the gases being pumped. One known way in which this can be achieved is by the use of a valve in the foreline which can open the foreline to atmosphere. This arrangement, however, risks potential contamination of the foreline with atmospheric moisture and hazardous process gases escaping to atmosphere. Alternatively, clean dry purge gas (such as nitrogen) can be injected into the foreline to relieve the vacuum, however, the volume of nitrogen required can be substantial, thus further increasing the cost of the system.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.